Patent GB-1480506-A (GB)
Title: Direction finders
IPC: G01S3/20
Filed: 1974-08-29
Abstract: 1480506 Radio direction-finders A E ISACSON 29 Aug 1974 37846/74 Heading H4D A direction finder includes: an antenna system arranged to receive signals and provided with a main antenna and with right-hand and left-hand lobe systems; coupling means by means of which said lobe systems can be alternately operative; a first comparison means for comparing the. signal intensities in the righthand and left-hand lobe systems with the signal intensity received from the main antenna; a first indicator arranged to be activated as soon as the signal intensity of the main antenna exceeds the signal intensity of one of the lobe systems; a second comparison means for comparing signal intensities-received on the righthand and left-hand lobe systems; and a second indicator arranged to indicate the output signals of said second comparison means; the righthand and left-hand lobe systems being arranged to partly overlap each other so that the maximum common signal intensity received exceeds the corresponding signal intensity received on the main antenna in the same direction, and the other common signal intensities received being less than the corresponding signal intensities received on the main antenna in the same direction.  As described, the aerial system comprises a central vertical dipole active element, and two sets of parasitic elements, each comprising a director and a reflector with central switches, so that either the one or the other set may be brought into effective operation as required. Thus three radiation patterns are available, which are utilized sequentially, viz., an approximately omni-direction pattern c, Fig. 5, (central element only), a pattern a with a major L.H. lobe (central element plus first set of parasitic elements), and a pattern b with a major R.H. lobe (central element plus second set of parasitic elements). An indicator 3, Fig. 3, is fed by the comparison means for the signal intensities received on the two lobed patterns a and b. When these intensities are equal, the needle 3a of said indicator lies in a central position and, provided the signal intensity received on the pattern c is less, it is known that the transmitting station 1 is in line with the intersection of the main lobes of the patterns a and b (0 degrees in Fig. 5) and its direction may be read on a compass 4. If the station 1 is to one side of said intersection, the needle 3a is deflected from the central position (Figs. 1, 2, not shown). Owing to back lobes a<SP>1</SP>, b<SP>1</SP> of the radiation patterns a, b, other equal signal intensity positions will also be indicated, for example, as shown in Fig. 5, at 110, 180 and 250 degrees. However, since in these positions the signal intensity received from the pattern c is greater, the other comparison means causes a warning flag 3b, Fig. 4, to appear. When such a warning is given, the aerial system should be rotated through 180 degrees. In the directionfinder a pulse generator 21, Fig. 7, controls the switching of the aerial system 2 and of the associated comparison means 30, 31. Power is supplied by a battery 32. The aerial system is connected with a frequency changer 22, which has a local oscillator 23, and which feeds a receiver 24. A load-speaker 25 is connected to an audio frequency output of the receiver and a limiter 26 is connected to a detected output thereof. The output from the limiter 26 is coupled through an amplifier 27 to two rectifiers 28, 29 which are connected respectively to the comparison means 30, 31. The means 30 compares the intensities of the received signals from the radiation patterns a and b with each other. The means 31 compares the intensities of the received signals from each of the radiation patterns a and b with that from the radiation pattern c. The aerial pattern switching sequence is a, c, b, c, &c., and when pattern c is effective no input is coupled to the comparator 30 (Fig. 6, not shown). The two comparators provide outputs to the indicator 3. Instead of operating a warning flag, output from the comparator 31 may be arranged to active a device which causes the needle of the indicator 3 to vibrate.